Did you ever wonder why there are no high-flying, wall-climbing, tall-building-leaping superheroesóor why there are no giant mutant monsters for them to fightóin real life? Find out what keeps big animals (like us) from engaging in astonishing feats of strength and ability and why there would be a downside to being tiny and all-powerful. The team that gave us What's Eating You? and Extreme Animals use chatty text and cartoon illustrations in this book for readers 8 to 12 to describe why humans can't fly, yet how some birds can travel 20,000 miles every year.

"This unique title uses geometry to discuss the varying strengths and capabilities of different animals. In a chatty tone, [Nicola] Davies starts off by describing the strict rules that control what bodies can and cannot do. She goes on to explain how the strongest animals are much smaller than humans. 'Some important features of bodiesólike how much food and air they needódepend on volume and weight. Othersólike the strength of musclesódepend on cross section or surface area.' This is the basis for the BTLT (Big Thing, Little Thing) Rule: 'If you DOUBLE the length of something, its surface area and cross section go up FOUR times, while its volume and weight go up EIGHT times!' BTLT is used throughout to explain why humans cannot fly, yet Arctic terns can travel 20,000 miles every year from pole to pole and back again. Humans cannot lift buses, but the rhinoceros beetle can lift 850 times its own weight."óSchool Library Journal